Abstract
Future changes in high latitude shrubification are expected to amplify warming. However, the elevational range and climate sensitivity of shrubification is poorly constrained, which hinders our ability to tune Earth system models. Using a sediment record from a high-elevation lake in Iceland (422 m asl), we leverage ancient birch DNA and mean summer air temperature reconstructed from bacterial lipids to constrain the climate sensitivity of highland shrubification. Our data show that during the Holocene Thermal Maximum, summer temperatures were 2.3 oC warmer than present and birch was present in the lake catchment, >150 m higher than its current regional limit. Subsequently, climate remained the dominant Holocene control on Icelandic birch cover at this site, even after human settlement – making our data ideal constraints for Earth system models. Using Holocene Thermal Maximum warmth as a partial analog for future climate change, ensemble climate projections suggest that the re-expansion of birch to higher elevations is likely by 2100 CE.
Acknowledgements
We kindly thank Sveinbjörn Steinþórsson and Þór Blöndahl for lake coring assistance, Thomas Marchitto for access to the Trace Metal Lab at the University of Colorado Boulder, and Diana Brum de Silveira, Brooke Holman and Isiah Castro for lab assistance.
Funding
National Science Foundation ARCSS #1836981 (GHM, ÁG, JS, and TT).
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Harning, D.J., Sacco, S., Raberg, J.H. et al. High elevation shrubification in the subarctic during the Holocene Thermal Maximum and 21st century.
Commun Earth Environ (2026). https://doi.org/10.1038/s43247-026-03965-3
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DOI: https://doi.org/10.1038/s43247-026-03965-3
Source: Ecology - nature.com
